The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in the specification should not be construed as an admission that such art is widely known or forms part of the common knowledge in the field.
Conventional flotation devices typically include a tank for receiving and containing slurry from a grinding mill, cyclone separator, or the like. The slurry is typically composed of mineral and other particles in a fluid suspension. The slurry is generally agitated within a tank of the flotation device and an aeration system is used to create air bubbles in the tank. Suitable reagents known as “collector reagents” are also added, which coat the surfaces of the valuable mineral particles within the slurry to make these particles hydrophobic and thereby to preferentially promote bubble to particle attachment. Other chemicals known as “frother reagents” are also added to assist in the formation of fine bubbles and support a stable forth zone at the top of the tank.
As bubbles rise toward the surface of the tank, they carry with them floatable valuable mineral particles, which form a mineral enriched surface froth. The froth then migrates over a lip and into a launder whereby the valuable mineral particles suspended in the froth are recovered from the tank as a mineral concentrate. The gangue particles remaining suspended in the slurry, along with those mineral particles that were not removed by flotation, are continuously discharged from the tank through a bottom outlet or other tails outlets.
One type of flotation device developed by the applicant for a grinding circuit is a skim air flotation device or cell, which receives the underflow stream from a cyclone device and recovers liberated valuable minerals from the recirculating load or feed stream. The skim air flotation cell uses a mechanical agitator, in the form of a rotor housed within a stator, normally disposed within the tank and driven by a motor and drive shaft, to agitate the slurry. An aeration system is also provided to direct air under pressure into the agitator through a central conduit formed within the drive shaft. Typically, the solids in the skim air are suspended using the agitator that also disperses the air into fine bubbles within the confines of the cell tank. These bubbles are typically in the range of 1 to 2 mm in size. The feed solids are fed at an inclined angle into the lower section of the flotation device so that the top half of the flotation device remains relatively quiescent and at a much lower pulp density than in the bottom half of the flotation cell.
The grinding circuit operates hydrocyclones which separate the mineral particles by centrifugal force and not by size. Hence, heavier minerals tend to build up in the grinding circuit and will be ground much finer if the skim air flotation cell does not recover them. At present, the skim air flotation cell is limited to recovering liberated valuable mineral particles in the normal range of flotation, typically in the very fast floating range of 38 to 106 microns. This means that the recovery of valuable mineral particles reduces as the particle size increases above 106 microns.
The primary aim of the skim air flotation cell is to remove these fast floating valuable particles and produce a concentrate that is capable of being cleaned to “final grade”. This is often achieved by using a dedicated flash cleaner cell that re-treats the rougher concentrate from the skim air flotation cell to produce a final grade concentrate. A flash flotation circuit may treat about 40 to 50% of the recirculating load so it sees a lot more of the valuable material in the recirculating load stream compared to a gravity device or hydrofloat device operating off a bleed stream from the mill discharge hopper, which must be initially pre-screened. Thus, the amount of valuable mineral particles recovered in a skim air flotation cell can be significantly higher. The residence time in a skim air flotation cell is relatively short and is not normally longer than 2 to 4 minutes. This is usually adequate as the skim air flotation cell is recovering the very fast floating liberated valuable particles in the 38 to 106 micron range.
Another type of flotation device, called a “hydrofloat”, may also be used for a grinding circuit to recover coarse valuable particles from the slurry. The hydrofloat is typically used to recover industrial minerals, such as phosphates, potash, graphite, coal, silica and iron ore sands, having a coarse size range up to 2 to 3 mm in size. The hydrofloat device has no moving parts or agitator, unlike the skim air flotation cell or other mechanical flotation devices and the like. Instead, the hydrofloat device operates primarily by forming a fluidised bed or teeter bed. A pump that supplies process water to the hydrofloat device uses a device to produce cavitations and fine microbubbles in the range of 0.2 to 0.5 mm in diameter, the microbubbles attaching to solid particles in the hydrofloat device. The fluidised bed is maintained by having air bubbles added in the range of 1 to 2 mm in size and these bubbles can attach themselves to the microbubbles already on the surfaces of the valuable mineral surfaces. The bubbles, microbubbles and attached solid particles then slowly rise up through the teeter bed because of their added buoyancy and overflow the hydrofloat device into a collection launder. The tails or rejects discharge from a conically shaped bottom of the device.
A typical hydrofloat device is 3 metres in diameter and has a water flow rate of 150 m3/hr of water, thus requiring its pump to draw 150 kW of power (i.e. 1 kW/m3). Hence, one disadvantage of a hydrofloat device is that it uses a substantial amount of energy to operate and is well within the typical range of power used by mechanical flotation devices even though it operates without a mechanical agitator. The hydrofloat device is also limited in that it can only accept a narrow range of particles so the feed material or slurry must be screened beforehand so as to only be in the range of 0.3 to 3.0 mm. In contrast, a skim air flotation cell can accept feed material or slurry up to 25 mm in size and hence requires no screening of the feed prior to treatment.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. It is also an object of the present invention in one preferred form to provide a method and apparatus for treating a feed stream of a flotation device so that it can process feed material of up to 25 mm in size and recover valuable solids particles larger in size than particles in the very fast flotation range.